The N I (5200 Å) dayglow

Simultaneous measurement of the 5200-Å dayglow from the N I (4S-²D) transition, neutral, and ion densities and photoelectron fluxes by Atmosphere Explorer is used to examine the production and loss mechanisms of N(²D) atoms. The branching ratio for production of N(²D) from N2+ + e is found to be larger than the branching ratio for the sum of the processes NO+ + e, N2+ + O, and e + N2. If it is unity, then the effective branching ratio for their latter processes lies in the range 0.34–1.0; the rate for quenching of N(²D) by O lies in the range 1.4–6.3 × 10−13cm³ s−1; the rate for the reaction of N(²D) with O2 lies in the range 0–2.7 × 10−12 cm³ s−1; and the rate for quenching of N(²D) by electrons is ∼1 × 10−9(Te/300)1/2 cm³ s−1. Values nearer the upper ends of these ranges are favored by a comparison between calculated and measured NO. The reaction N+ + O2 → N + O2+ is a small source of N in the thermosphere. A calculation shows that the ratio of N(4S) to O is 0.01–0.02 at 250 km.